This article presents the development of a junior version of the Spanish (Castilian) NEO Personality Inventory-Revised (JS NEO) suitable for adolescents aged 12 to 18 years. The psychometric properties of the new JS NEO were investigated using two samples of 2,733 and 983 adolescents in Spain. The results showed that the adult NEO-PI-R factor structure was replicated with the junior version of the inventory and that the reliabilities of the scales were adequate. The cross-form correlations between the junior and the adult versions of the questionnaires indicated good equivalence indices. Furthermore, a joint factor analysis of the JS NEO and the Big Five Questionnaire-Children (BFQ-C) provided additional evidence for the construct validity of the JS NEO. (Contains 8 tables.)